A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the ossicles of the middle ear 103 that vibrate the oval window 106 and round window 107 membranes of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the cochlear nerve 105 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted by the cochlear nerve 105 to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear, a conventional hearing aid or a middle ear implant may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
Middle ear implants employ electromagnetic transducers that convert sounds into mechanical vibration of the middle ear 103. A coil winding is held stationary by attachment to a non-vibrating structure within the middle ear 103 and microphone signal current is delivered to the coil winding to generate an electromagnetic field. A magnet is attached to an ossicle within the middle ear 103 so that the magnetic field of the magnet interacts with the magnetic field of the coil. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear 103. See U.S. Pat. No. 6,190,305, which is incorporated herein by reference.
U.S. Patent Publication 20070191673 (incorporated herein by reference) describes another type of implantable hearing prosthesis system which uses bone conduction to deliver an audio signal to the cochlea for sound perception in persons with conductive or mixed conductive/sensorineural hearing loss. An implanted floating mass transducer (FMT) is affixed to the temporal bone of the skull. In response to an externally generated electrical audio signal, the FMT couples a mechanical stimulation signal to the temporal bone for delivery by bone conduction to the cochlea for perception as a sound signal.